The researchers used a Californian study of 118,094 people and focused on 35,561 who had never smoked but were married to someone who was a current or former smoker.

The researchers, James Enstrom of the University of California and Geoffrey Kabat of New Rochelle in New York, found that among smokers there was a strong risk of heart disease, lung cancer and chronic obstructive pulmonary disease.

But exposure to passive smoke, through being married to a smoker, was not significantly linked with death from heart disease or lung cancer.

They said: "These findings suggest that the effects of environmental tobacco smoke, particularly on coronary heart disease, are considerably smaller than generally believed."

In an accompanying editorial, Prof George Davey Smith of Bristol University said it was difficult to measure the effects of passive smoking and so the risks could be distorted.

He said the study findings suggested that passive smoking could substantially increase the risk of chronic obstructive pulmonary disease, and warned the researchers may have over-emphasised the negative nature of their findings.

The BMJ said James Enstrom had received funds from the tobacco industry for research, because it was impossible to get the money from other sources.

Geoffrey Kabat said he had never received money from the industry until last year, when he conducted a review for a law firm which has several tobacco companies as clients.

The BMJ states: "They are both lifelong non-smokers whose primary interest is an accurate determination of the health effects of tobacco."

But Amanda Sandford, from anti-smoking group Ash, said: "The authors appear to be deliberately down-playing the findings to suit their tobacco paymasters."

Simon Clark, of the smokers' lobbying group Forest, said: "We have consistently argued that the jury is still out on the effects of environmental tobacco smoke. This latest study proves our point."

The British Medical Association criticised the study, because it only re-analysed a small part of data from a study that was dropped by its original funders, the American Cancer Society.

The BMA said most of the data has been around for decades, but was judged by many expert groups to be inadequate to accurately measure passive smoking.

Tim Lord, chief executive of the Tobacco Manufacturers' Association, said: "The debate on environmental tobacco smoke is far from over, contrary to what many people say."

Hardcore elderly

Nearly a third of pensioners who smoke are "hardcore smokers" with no interest in giving up, according to research published today.

Many have the kind of attitude displayed by the EastEnders character Dot Cotton, who is rarely seen without a cigarette, said Cancer Research UK, which carried out the study.

The research in today's British Medical Journal was based on interviews with 7,766 adult smokers in England.

It defined "hardcore" smokers as those who have had less than a day without cigarettes in the past five years, have made no attempt to give up in the previous 12 months and have no desire to quit.

The study found just 5pc of smokers aged 16-24 could be classed as hardcore - but 30pc of those aged 65 and above fell into this category.

A third of hardcore smokers thought their health was totally unaffected by the habit and 31pc said smoking was the main pleasure in their life.